This invention relates to an apparatus having a calibration field and a method for measuring internal parameters, such as a principal point position, a screen distance (focal length) and a distortion parameter of a lens, necessary to correct an image distortion of a camera and particularly relates to an apparatus and method for calibration suitable for use in calibration of a camera having a zoom lens.
Conventionally, in the field of photogrammetry or photographic measurement it has been important to obtain an image with less aberration. For this purpose, in these fields a high-accuracy lens with small aberration has been used for a photographic camera. Further, in the field of photogrammetry, multiple points positioned in precisely measured three dimension are measured in plural directions and then the internal parameters, such as a principal point position, a screen distance (focal length) and a distortion parameter, of a camera are analytically obtained, as disclosed in JP-A-9-329418. On the other hand, in the field of photographic measurement, the internal parameters of a fabricated camera are obtained by precisely measuring the camera.
These methods for obtaining the internal parameters of a camera are adapted to calibrate a fixed-focus camera. In general, objects to be measured may have a variety of sizes and there may exist an obstacle to a measurement work. In such cases, for the fixed-focus camera, the object distance thereof is limited. Thus, the fixed-focus camera is given merely an inappropriate angle, resulting in a degraded accuracy of measurement, for example. To the contrary, in a camera provided with a zooming function, such as a modern digital camera and video camera, its focal length can be variably adjusted. Accordingly, a selection of an appropriate focal length for a zoom lens will allow to take an appropriate image of an object to be photographed even if there is an obstacle which limits the object distance.
However, the focal length of the zoom lens is variable within a range from a zooming area to a wide-angle area and a calibration work must be therefore repeated for respective focal lengths. It is therefore a problem that mere application of the method suitable for the fixed-focus camera to obtain internal parameters thereof to the zoom lens complicates the calibration work thereon. In other words, although the focal length of the zoom lens can be adjusted in both ranges of a wide-angle side and a zooming side, the position where an image is taken must be changed corresponding to each change of the focal length. This complicates calibration works. When the focal length of the zoom lens is set within the zooming side, it is required to fully secure a distance between the camera and a three-dimensional field for calibration in which a number of points measured precisely and distributed three-dimensionally are included. It is therefore a problem that an extensive space is required for calibration works. When the focal length of the zoom lens is set within the wide-angle side, a wider structure is needed as a three-dimensional field. Accordingly, it is also a problem that construction cost of the three-dimensional field is necessarily increased.